Coatings are generally applied to solid particles, such as pharmaceutical forms, to protect the ingredients against the atmosphere, to mask unpleasant tastes and odors, to ease in swallowing, to improve the appearance as well as providing coloring and printing.
Methylcellulose and hydroxypropyl methylcellulose have been used for a long time as coating materials for pharmaceutical forms. U.S. Pat. No. 3,431,138 discloses that these coating are tacky, uneven, and require extensive polishing after coating. To solve these problems, the U.S. Patent suggests a coating composition which comprises from 50 to 60 weight percent of ethanol, from 35 to 45 weight percent of chloroform and from 2 to 5 weight percent of low viscosity methyl cellulose. Since the issue of the U.S. patent, the coating technology has progressed and high quality coatings are obtainable without the use of chloroform. Nowadays methylcellulose and hydroxypropyl methylcellulose are dissolved in water or a mixture of water and alcohol and sprayed on an agitated mass of pharmaceutical forms. The spraying technique is a sophisticated process which requires well-defined processing parameters and quite complex equipment Moreover, the viscosity of the solutions of methylcellulose and hydroxypropyl methylcellulose must be low enough that they are still sprayable.
U.S. Pat. No. 3,607,364 discusses in detail the disadvantages of spray coating of pharmaceutical solid forms, such as the high pressures which are required to sufficiently atomize a coating medium. To solve these problems, U.S. Pat. No. 3,607,364 discloses a process for coating a pharmaceutical solid form wherein a foamed viscous sugar medium is applied to the solid surface, the coating medium is then urged against the solid form surface to break down the foam and produce an even coat of the coating medium on the solid form surface. Unfortunately, the examples reveal that this method requires a lot of work and time. 15 to 20 coats were required to obtain elegant coated tablets.
U.S. Pat. No. 4,965,089 relates to a method and apparatus for coating caplets with gelatin. There are two primary methods of applying a liquid gelatin composition to a pill. The first method is to hold the pill or tablet during a dipping process. Unfortunately, quite sophisticated and expensive equipment is necessary to provide an inefficient dipping process that suffers from long processing times and product variability. The second method is to spray a coating over the pill as it tumbles.
In view of the deficiencies of the prior art processes, it is an object of the present invention to provide a new process for coating solid particles.